1. Field of the Invention
The present invention relates to an antenna, a high-frequency module mounting the same, or a radio terminal that are applied to a radio communication-related equipment for providing a user with a radio communication system service, such as satellite broadcasting, global positioning system (GPS) using a circular polarized wave, in more particularly, to a small-sized thin type distributed phase type circular polarized wave antenna, a high-frequency module including the antenna, and a radio terminal mounting them, which is suitable for providing the user with information transmission radio communication system by the medium of electromagnetic wave having a wavelength greater than dimensions of the radio terminal.
2. Description of the Related Art
Among various radio communication system, many satellite-using systems such as seamless international telephone, satellite broadcasting, GPS, are operated, by making full use of advantages thereof, e.g. a seamless services over different countries can be provided, and a shielding effect of tall structures is small, since an electromagnetic wave used as a communication medium is transmitted from a substantially vertical (zenith) direction.
On one hand, the seamless services can be provided internationally. On the other hand, a possibility that the electromagnetic wave is leaked to other countries and other regions is inevitably highs so that different polarized waves (right-handed circular polarized wave and left-handed circular polarized wave) are assigned to neighboring countries and neighboring regions by using circular polarized wave, so as to solve the problem of electromagnetic wave leakage. The right-handed circular polarized wave cannot be received by a left-handed circular polarized wave antenna, and the left-handed circular polarized wave cannot be received by a right-handed circular polarized wave antenna. Only a half power of the circular polarized wave can be received by a linear polarized wave antenna. Therefore, so as to provide effectively the user with a radio communication services using the electromagnetic wave of a circular polarized wave, means for realizing the circular polarized wave antenna becomes an important technical problem.
As the means for realizing circular polarized wave antenna, two methods are conventionally known and are put to practical use.
A first conventional method is to dispose two linear polarized wave antennas orthogonally to each other, and feeding phases of the respective antennas are shifted by 90°. A cross dipole is well known as a representative example of the first conventional method, as shown in “Illustrated antenna (zusetsu antenna)” by Naohisa Goto, 1995, Institute of Electronics, Information and Communication Engineers, page 219. However, in the first conventional method, two power feed parts are required, and means for shifting the respective power feed parts by 90° (e.g. phase converter) are further required. In the first conventional method, there is a disadvantage in that a circuit size of a radio communication device using the antenna is enlarged, so that there is problem in miniaturization of the radio communication device.
A second conventional method is to use a periphery-opened patch antenna such as a microstrip antenna, namely, to realize a circular polarized wave antenna with a single power feed point by using a rectangular or circular two-dimensional patch, which extends along two axes orthogonal to each other. For example, as shown in “Small size plane antenna” by Misao Haneishi et al, 1996, Institute of Electronics, Information and Communication Engineers, pages 143 to 145, a regular square or circle is such deformed that one side is shorter and another side is longer along the two axes orthogonal to each other. As a result, a length of one side of the regular square or a half circumference length of one side of the circle is made different from another side, and the length of each side is slightly shorter or longer than ½ wavelength of the receiving wavelength. Viewed from a power feed point, the length of the side along the respective axes orthogonal to each other functions as inductance or capacitance, and a feeding phase to the length of the side of the respective axes is shifted by 90°. The second conventional method is more advantageous than the first conventional method, since only the single power feed point is provided and a circuit size of a high-frequency circuit for supplying a high-frequency power to the antenna can be significantly reduced. Therefore, the second conventional method is actually most commercialized.
However, when using the second conventional method, two-dimensional size of substantially ½ wavelength of the radio wave received by the antenna should be assured as outer dimensions of the antenna, namely, an area of a regular square having one side of substantially ½ wavelength should be assured. Accordingly, there is an obstacle for application to a palm sized small terminal that is currently desired.
So as to reduce the dimensions of the antenna according to the second conventional method, a technology for miniaturizing an antenna by using a wavelength compact effect of a dielectric material, in which the antenna is lined with or covered with a dielectric material having a high dielectric constant.
However, another problem in miniaturization is occurred, for example, a fabrication cost is increased by using the dielectric material having the high dielectric constant, and a dimension of the dielectric material in a thickness direction is increased so as to mostly produce the wavelength compact effect of the dielectric material.